1. Field of the Invention
This invention relates to mirror thermal insulation and more specifically to mirror insulation having an adjustable dimension to permit use of a standard component to fit a nonstandard dimension.
2. Description of the Prior Art
The use of bright, highly reflective metallic sheets to provide thermal insulation for industrial equipment is well known. The high reflectivity of the typical aluminum or stainless steel sheets used for this insulation reduces radiation of heat from heat sources, such as pipes, panels and etc.
Mirror insulation normally comprises a plurality of parallel spaced layers, which reduces heat loss through convection and reduces the total amount of heat radiated from the entire assembly. It is important to reduce the size and number of gaps in the total insulation package to minimize convective heat loss.
Mirror pipe insulation is commonly used in the nuclear power industry, where it may be subjected to radioactive environment. In such an environment it offers substantial benefits over conventional thermal insulation, which comprises blocks of refractory material or low conductivity masses of fibrous material such as glass fibers, mineral wool, etc., because multilayered mirror insulation is much stronger and the open structure of the insulation permits easy cleaning of its interior. These advantages coupled with efficient insulating properties have lead to enthusiastic commercial acceptance of reflective mirror insulation, particularly in the nuclear power industry.
Commercial use of such insulations, however, has also created serious problems of fabrication and installation, resulting from the very structure of the insulation itself. Fibrous or ceramic insulations are generally cut to fit pipes or other objects to be insulated on the job site with simple tools such as portable saws. The complex metallic structure of multilayer mirror insulation, in contrast, virtually requires that it be fabricated in a sheet metal shop where tolerances and quality can be controlled more closely and then transported to the job site for installation. Naturally, production efficiency is higher when such insulation is manufactured in standard sized units. Sometimes, however, reflective mirror insulation is custom designed and built from construction drawings of the objects to be insulated supplied by the builder. Even then, however, the insulation units do not exactly fit because the field structures do not conform exactly to the engineering drawings. In either case a unit of reflective mirror insulation must be completely built especially for the nonstandard dimension required, or modified prior to installation.
It has been suggested that this problem of misfitting parts could be resolved by working directly from dimensions taken in the field. Taking such dimensions is, however, extremely time consuming and substantially delays completion of construction projects because fabrication of reflective mirror insulation according to this method cannot begin until the structure to be insulated is complete.
The problem also hinders attempts to design and manufacture standard sizes of insulation. At least some sections of insulation must be individually tailored to a particular size, placing reflective insulation at a disadvantage compared to ceramic or fibrous insulations that can be stockpiled and readily cut to size on the job site. Consequently, it would be of considerable benefit to provide a reflective mirror insulation whose size can be adjusted.
One effort in this direction is disclosed in U.S. Pat. No. 3,892,261 to Hoeman. Hoeman '261 discloses a multilayered reflective mirror insulation for a pipe. The length of the insulation is expandable. Expansion is achieved by pulling each end of the insulation sheet. Each layer of insulation includes an adjustable mid-section comprising a splice piece set composed of two spaced parallel metal sheets fixed to one layer to form a female extension member. The corresponding abutting layer from the other end of the adjustable section slips between the two layers of the splice piece set, forming a sandwich. The sandwich construction was designed to reduce convection around the joints formed where the respective layers abut and to increase the rigidity and strength of the structure. Only the frictional engagement of these layers and splice piece sets holds the layers in their proper parallel spaced relationship throughout the length of the adjustment. Adjustable mirror insulation units according to this invention were built, but were employed sparingly because manufacture of the unit is extremely labor intensive and expensive since it calls for many welds and very close tolerances. In addition, convective heat loss through the joints was much greater than anticipated or acceptable because mere frictional engagement in the multilayered sections was not sufficient to maintain proper spacing between adjacent layers and maintain a relatively air tight seal at the interface of a layer and a mating splice piece set. Maintaining proper alignment of these joints when the insulation is rolled to enclose a pipe is exceedingly difficult.
A significantly later attempt to address these problems is disclosed in U.S. Pat. No. 3,904,379 to Oser. Oser '379 likewise discloses an adjustable multilayered mirror pipe insulation. In Oser '379 each layer is kept in its proper parallel spaced relationship to the others by cone shaped projections or stand-offs. In the adjustment portion, however, only the frictional engagement of abutting overlapping layers and the inherent rigidity of each layer maintains the proper spaced relationship, as well as the frictional engagement of the overlapping layers. A plurality of bolts penetrates all layers of insulation in the adjustment section to secure the insulation in its adjusted position after installation. To permit pull-apart adjustment with the bolt in place, and to prevent pulling the insulation completely apart, each layer includes a longitudinal slot. Although Oser '379 discloses a simpler structure than Hoeman '261, Oser '379 suffers from similar difficulties. The bolts that penetrate all layers of the insulation conduct much heat from the heat source by providing a direct path from the heat source to the ambient air. Slots throughout the length of permitted adjustment invite large convective heat losses. Similarly, the direct contact between overlapping insulation layers is only maintained by the rigidity of the layers themselves. In installation and operation they are extremely likely not to retain the neat precisely aligned arrays illustrated in the drawings, but to separate moderately, permitting further convective heat loss. The largest single disadvantage of such adjustable mirror insulation is the extraordinarily high heat loss in the adjustment section. Oser '379 patent suffers more from this defect than other prior art solutions to these difficulties.
Furthermore, in these prior art approaches, the insulation could only be expanded. Only with great difficulty and resultant substantial reduction in insulating properties could the insulation be contracted. Thus, if workers expanded a unit of adjustable insulation too much to fit in a given application, it could quite likely be ruined and require complete replacement.
Accordingly, there is a clear need for an improved adjustable multilayered reflective mirror insulation that can be expanded or contracted, that is less labor intensive to manufacture and thermally insulates better, and withstands mishandling during shipment and installation better than the prior art.